With the improvement on computer power in recent years, simulation techniques have also gradually developed. Consequently, simulations have been used in various fields of application.
As numerical calculation techniques of solving problems of continuums such as fluids and elastic bodies, a finite difference method, a finite element method, a finite volume method, and so on have been often used to determine an approximate solution of a differential equation on the basis of a lattice. In recent years, such numerical calculation techniques have been developed to utilize numerical calculations in fields of application such as the Computer Aided Engineering (CAE), thereby solving a problem of an interaction between a fluid and a structure.
However, in techniques that use a lattice, such as the finite element method and the finite volume method, when there is a problem of the presence of an interface such as a free surface or a problem of the occurrence of a moving boundary such as a fluid-structure interaction problem, such a problem is dealt with in a complex way, and hence it is often difficult to create a program.
By contrast, in particle methods that do not use a lattice, such as the Moving Particle Semi-implicit (MPS) method and the Smoothed Particle Hydrodynamics (SPH) method, special treatments are not used to deal with moving boundaries. Thus, the particle methods have been widely used in recent years.
The particle methods have been developed to readily deal with a boundary that moves and is deformed, e.g., a free surface. However, when a continuum becomes a simple particle group due to a discretization method, it becomes unclear where a boundary of a continuum is, as depicted in FIG. 1. Accordingly, the particle methods do not have a unified technique for solving a problem wherein a boundary such as surface tension is explicitly dealt with, and there are some technologies separately developed (see for example patent document 1, non-patent documents 1, 2, and 3).
For example, Patent document 1 describes a technique wherein a potential is configured from an interparticle distance so as to cause an attraction between particles, as depicted in FIG. 2. This is a technique created on the basis of the idea that surface tension results from nonuniformity of an intermolecular force on a surface.
In the techniques of non-patent documents 1, 2, and 3, on the assumption that particles with a decreased number of neighboring particles are particles at a boundary, surface tension is added to the particles at the boundary.
Non-patent documents 2 and 3 introduce a model for achieving wettability.    Patent document 1: Japanese Laid-open Patent Publication No. 2008-111675